1. Field of the Invention
The invention generally relates to a filter and a filter method, and more particularly to a filter and a filter method of an optical storage system.
2. Description of Related Art
With regards to an optical storage system, a servo controller adjusts characteristic parameters as well as performs data determinations according to a focus error (FE) signal, a tracking error (TE) signal, and a radio frequency (RF) signal generated by a photo sensor on an optical pickup head. The RF signal generated by the photo sensor is first processed by a filter, then provided to a back-end data loop, so that a modulated signal is demodulated.
FIG. 1 is a schematic block diagram of a filter for a conventional optical storage system. Referring to FIG. 1, a filter 100 includes a direct-current (DC) tracking unit 110 and a subtracter 120. During filter operation, a DC tracking unit 110 extracts a DC component of a RF signal RF_IN1 detected by the photo sensor, so as to generate a level signal RF_DC1. Thereafter, a subtracter 120 subtracts the level signal RF_DC1 from the RF signal RF_IN1, so as to generate a RF coupling signal RF_OUT1. That is, RF_OUT1=RF_IN1−RF_DC1. Accordingly, a back-end data loop 101 demodulates the RF coupling signal RF_OUT1, so as to generate a modulated signal.
It should be noted that, FIG. 2 is a timing diagram of the signals for the filter depicted in FIG. 1. Referring to FIGS. 1-2, when a variation in a DC level of the RF signal RF_IN1 is generated, for instance at a time reference t11 shown in FIG. 2, the DC tracking unit 110 is limited by a bandwidth thereof, and the DC component may not be extracted in time. Moreover, when the RF signal RF_IN1 returns to normal in a next instant, for instance at a time reference t12 shown in FIG. 2, the DC tracking unit 110 cannot react in time. Consequently, the level signal RF_DC1 cannot return to a normal level instantaneously. At this time, a variation phenomenon of the RF coupling signal RF_OUT1 occurs at a label 210 in FIG. 2, for example.
In other words, to prevent conventional filters from generating a large jitter after a normal RF signal RF_IN1 is processed by the DC tracking unit 110, the bandwidth of the DC tracking unit 110 is typically kept low. However, when the aforementioned phenomenon occurs in the RF signal RF_IN1, the RF coupling signal RF_OUT1 is usually distorted due to the insufficient bandwidth of the DC tracking unit 110. Therefore, a disk reading ability of the optical storage system may be affected.